1. Field of the Invention
The present invention is directed to a method for compensating eddy currents caused by gradients in a nuclear magnetic resonance apparatus.
2. Description of the Prior Art
As is known, a topical resolution of the nuclear magnetic resonance signals in nuclear magnetic resonance tomography ensues by superimposing a magnetic field gradient on a homogenous, static basic field on the order of magnitude of 1 T. The principles of such imaging are set forth, for example, in the article by Bottomley, "NMR-Imaging Techniques and Applications: A Review", in Review of Scientific Instrumentation, 53 (9), 9/1982, pages 1319 through 1337.
For topical resolution in three dimensions, magnetic field gradients must be produced in three directions that preferably reside perpendicularly relative to one another. A coordinate cross x, y, z is entered in FIG. 1 and in FIG. 2 herein, representing the directions of the respective gradients. FIG. 1 schematically shows a conventional arrangement of gradient coils for generating a magnetic field gradient Gy in y-direction. The gradient coils 2 are executed as saddle coils that are secured on a carrying tube 1. A substantially constant magnetic field gradient Gy in the y-direction is generated within a spherical examination volume 4 by the conductor sections 2a. The return conductors--due to their greater distance from the examination volume 4, produce only negligible magnetic field components therein.
The gradient coils for the x-magnetic field gradients are identically constructed to the gradient coils 2 for the y-magnetic field gradients and are merely rotated on the carrying tube 1 by 90.degree. in the azimuthal direction. For clarity, they are therefore not shown in FIG. 1.
The gradient coils 3 for the magnetic field gradient in the z-direction are schematically shown in FIG. 2. The coils are annularly executed and symmetrically arranged relative to the center of the examination volume 4. Since the two individual coils 3a and 3b are traversed by current in opposite directions in the way shown in FIG. 2, they cause a magnetic field gradient in the z-direction.
The required magnetic field gradients must have steep leading and trailing edges and must be optimally constant during the on-time. As a result of the steep leading and trailing edges, however, eddy currents are induced in metallic parts of the nuclear magnetic resonance tomography apparatus, particularly in the inner hollow cylinder of the cryostat that surrounds the examination space. The eddy currents in turn generate magnetic fields that are directed oppositely to the magnetic field gradients. This leads to a rounding the corners of the desired square-wave pulses and also causes a parasitic magnetic field that decays after the magnetic field gradients are turned off. This is shown in FIG. 3, wherein the desired square-wave magnetic field gradient is referenced G.sub.s. The gradient curve actually achieved when an approximately square-wave current pulse is turned off is referenced G.sub.l.
U.S. Pat. No. 4,698,591 discloses deformation of the gradient current pulse with a filter such that the magnetic field gradient is ultimately largely approximated to the desired square-wave form. As shown in FIGS. 1 and 2, such a filter 7 or 10 is inserted between the drive circuit 6 or 9 and each of the respective gradient coils 2 or 3. These filters are high-pass filters. For determining the parameters required for the filter, the magnetic fields caused by the eddy currents must first be measured. This, for example, can ensue with a magnetic field probe with which the magnetic field actually generated is measured at various points in the examination space. To that end, however, a separate measuring instrument is required.
The aforementioned U.S. Pat. No. 4,698,591 discloses a method of measuring the magnetic field course with the nuclear magnetic resonance signals induced in a sample. Since the measurement of the magnetic field is required at at least two locations of the examination space, the sample must be moved back and forth between two measuring positions for every measuring cycle.